The invention relates generally to radio frequency (RF) communication devices and more specifically to an RF mixer circuit having signal splitting and phase shifting circuitry (xe2x80x9cbalunxe2x80x9d), wherein the balun is integrated on-chip with the core RF mixer circuit. The invention further relates to methods for making RF mixers integrated on-chip with one or more baluns.
Wireless communication via radio frequency (RF) wave transmission presents numerous technical challenges. Contemporary wireless products typically divide RF functions among several integrated circuits and discrete devices, and separate functions may be manufactured in different technologies. Large scale integration (LSI) is a desirable goal in virtually all electronic manufacturing processes but has proven difficult for RF applications, and consequently modular or hybridized approaches are typically used to manufacture products to accomplish complex functions such as frequency translation or xe2x80x9cmixing.xe2x80x9d
RF components such as mixer modules have generally been made by combining semiconductor technology, such as gallium arsenide transistors or silicon diodes, with passive component networks, all mounted on a common carrier. Such hybrid assemblies add cost and complexity to the manufacturing process by comparison with fully monolithic or integrated solutions, and modular construction of parts such as mixers can result in poorly performing, bulky modules that must be soldered on to an xe2x80x9cintegratedxe2x80x9d system. Combining on a single chip RF functions, passive components (e.g., resistors, capacitors, inductors), and the control functions, usually fabricated in CMOS circuitry on silicon, for example, could enhance product performance, reliability, and manufacturing flow tolerances, while decreasing size, power consumption, and manufacturing costs. However, attempts at high levels of integration which attempt to combine RF functions on a single chip have proven to be difficult, expensive and generally unsatisfactory.
One earlier approach to partially integrating several different functions on a single substrate is the thin-film hybrid process. The combination of several chips on a thin-film hybrid substrate for RF applications requires precise manufacturing practices and circuit elements. Many processing steps are necessary. For instance, these steps generally include depositing a thin layer of metal on a substrate, coating it, and finally removing the metal layer by etching to form a desired pattern. This process is repeated for the deposition of resistive films to create circuit elements. Ultimately, semiconductor devices are attached to the patterned substrate and these individual chips are interconnected by the patterned transmission lines within the hybrid package.
Such thin-film hybrid fabrication processes can be expensive and time consuming and can have significant yield problems that increase proportionally with the number of integrated circuits on the substrate. Furthermore, modular or hybrid construction processes can be economically inefficient due to the testing and retesting of the various subassemblies required to feed into the end product. The further along in the process before defects are discovered, the greater the waste of resources. Thus, yield losses from the incorporation of multiple chips on a hybrid circuit can be very costly.
Another factor which can increase hybrid cost is variation in stray coupling between closely-spaced RF circuits. This phenomenon is traceable to the placement of lumped element components. The mechanical alignment precision of these components on the hybrid substrate is inherently poor, their sizes and shapes are numerous and varied, and the placement equipment typically has loose registration tolerance for placement accuracy. Stray RF coupling between active components, passive components, and interconnect wiring can be a significant factor in reducing manufacturing yields. These stray coupling variations can make it difficult to achieve repeatable circuit performance, thereby resulting in serious yield problems.
For the above reasons, attempts to integrate systems more completely with monolithic microwave integrated circuits (MMICs) have increased. Many of these efforts have been frustrated, however, by the limited availability of high performance substrates. A number of significant problems arise from using substrates that are not highly insulating. High electrical loss, high inter-element parasitic capacitance, high conductor-to-substrate capacitances, and other deleterious effects can result from using substrates such as gallium arsenide (GaAs) and bulk silicon that are not highly insulating.
Substrates such as gallium arsenide (GaAs) and bulk silicon can have serious disadvantages for the integration of both active and passive RF components in a single chip. For silicon, for example, the performance of passive components can be severely impaired by the conductivity of the substrate. Insertion loss along transmission lines and isolation between non-connected devices are both poor owing to this conductivity. For GaAs, for example, the ability to integrate large numbers of active devices can be limited by a relatively high defect density of the substrate. Both technologies have their individual merits, but they cannot be merged readily into a single system except through modular methods.
Differential signal processing enhances performance in an RF system. Integrated circuit manufacturing naturally promotes differential design techniques because of the small size, low cost, and superior matching of devices available. For cost reasons, signal routing in the hybrid or module realm tends to be single ended rather than differential. RF signals tend to be routed through expensive cabling and precision machined RF connectors. Naturally, single ended routing cuts costs in half and is very desirable to endpoint manufacturers of RF equipment. Routing single ended signals into and out of differential circuits, on the other hand, introduces problems that must be overcome.
One known type of device for combining differential signals into a single ended output signal is referred to in the art as a xe2x80x9cbalunxe2x80x9d (balanced input/unbalanced output). A balun is often used when it is desired to couple a balanced system or device to an unbalanced system or device, or vice versa. A typical example is the coupling of a two-line (balanced) circuit, such as a cellular telephone transmitting circuit, to a single-line (unbalanced) circuit, such as an antenna. Another example is the use of a balun as a signal splitter/phase shifter used with a balanced mixer, wherein a single ended input signal is split into complementary signals that are 180 degrees out of phase with one another.
Conventional baluns are tightly coupled structures fabricated much like a conventional transformer that uses discrete components, e.g. typically comprising transformer-coupled windings on a ferrite core. When implemented as discrete components in modular design approaches, these baluns require a relatively large amount of board space.
In applications that are sensitive to size and accuracy, e.g., wireless mobile telephones, a balun must meet the criteria of compactness, minimum insertion loss and power wastage, and precise 0-180xc2x0 phase separation. Although prior art baluns are known which accomplish one or two of these objectives, there are no economical solutions which satisfactorily accomplish all three. Using discrete lumped element components, instead of transformers, to generate the complementary 0-180xc2x0 signals from a single ended source is an effective method, but it requires that the inductor and capacitor elements used in the networks match one another with high accuracy. This design approach argues strongly in favor of an integrated circuit solution wherein element matching can easily be better than 1%.
A mixer is a critical component of radio-frequency (RF) systems. It is usually the first or second device after the RF input, so the mixer is crucial to the operation of the system. Various mixer parameters, such as bandwidth, interport isolation, conversion efficiency, and linearity, must be optimized for each given application. The mixer translates an RF signal at one frequency into a signal at a different, usually lower, frequency, in order to make signal propagation easier and less expensive. Changing the frequency of a signal without altering its information content is necessary because signal processing components, such as amplifiers, are much less expensive and work better when they are designed to operate at lower frequencies.
A radio receiver mixer is generally a three port non-linear device that takes an incoming low level radio frequency (RF) signal and multiplies or mixes it with a strong signal from a local oscillator (LO) to produce signal frequencies including the sum, difference (IF) and cross-products of the RF and LO signals. Mixers, therefore, are employed in devices where it is desirable to convert a higher frequency signal to a lower frequency signal including any receiver systems such as wireless communications base stations and so forth. Mixers are also employed in devices requiring upconverting a low frequency signal to a higher frequency signal.
Mixing an input signal with a local oscillator (LO) signal yields upper and lower sidebands around the LO frequency. Each sideband has the same information content as the input signal. The upper sideband is the sum of the input and LO frequencies, while the lower sideband is the difference between the input and LO frequencies. Usually, it is the lower sideband (the xe2x80x9cdownconvertedxe2x80x9d signal) which is used in receiving systems, whereas the upper sideband (the xe2x80x9cupconvertedxe2x80x9d signal) is typically used in transmission systems. The upper or lower sideband, whichever is chosen, is called the intermediate frequency (IF) signal.
There are basically four types of mixers: single-ended, singly balanced, doubly balanced, and doublyxe2x80x94doubly balanced (also called triply-balanced). However, all types are three-port devices, and comprise an input port (the RF port), a local oscillator input port (the LO port), and an output port (the IF port). The LO, RF and IF ports combine through filters to provide some degree of inter-port isolation. Single-ended mixers generally have a narrow bandwidth, limited dynamic range, and poor inter-port isolation.
Better isolation and broader bandwidth can be obtained with a singly-balanced mixer. The mixer is fed by the LO signal through a balun which provides differential (180 degrees out of phase) signals to the mixer. Doubly balanced mixers feed the RF signal through a second balun, providing interport isolation both between the LO and RF ports and between the LO and IF ports. Doubly balanced mixers typically use twice the number of diodes or transistors (four) as a singly-balanced mixer, and the diodes or transistors are often, although not always, arranged in a xe2x80x9cquadxe2x80x9d ring configuration. A doublyxe2x80x94doubly balanced mixer ordinarily employs twice the number of diodes or transistors (eight) as a doubly balanced mixer and is often, though not always, realized by combining two quad ring mixers. The number of circuit nodes and conductors rises commensurately, and doublyxe2x80x94doubly balanced mixers thus engender complex circuit topographies.
The balanced mixers may additionally employ a balun to convert the differential IF output signals into a single ended IF output signal, depending on the requirements of the application.
Active mixers employ a current source and perform the mixing function in terms of currents. By contrast, passive mixers switch voltages. Considering that silicon CMOS technology offers excellent switches, high performance passive mixers are preferably realized in CMOS form. Passive mixers are compact and have the potential for extremely low power operation, making them well-suited for wireless communications applications, e.g. in mobile phones. Generally, the trade off that they providexe2x80x94high linearity and low noise in exchange for lower gain and less bandwidthxe2x80x94is acceptable in such end-uses.
The challenge presented RF components made in silicon CMOS technology is on-chip integration with other passive components such as inductors and capacitors. This challenge is substantial for RF mixers which require differential signal processing circuitry having capacitors and relatively sizable inductors that render the circuits susceptible to deleterious substrate coupling effects when formed on substrates that are not highly insulating. Examples of such substrates include GaAs, silicon, and even silicon-on-insulator (SOI) substrates, where a silicon substrate is isolated from the device Si layer by an intervening oxide layer.
To summarize, there exists a need to fabricate RF mixers that require differential signal processing in an on-chip integrated form that consumes low amounts of power and that provides high performance as measured by low noise and high linearity. It would be advantageous to form RF mixers that are wholly isolated, to avoid parasitics and losses due to substrate coupling. It would be further advantageous to reduce the cost, size, and manufacturing complexities of RF mixers by replacing discrete components with on-chip integrated circuit elements wherever possible. Significant system performance improvements could be realized if RF mixers, their required balun circuits, and any associated silicon-based CMOS control circuitry could be integrated on the same chip to produce a high-yield, high-performance RF mixer device.
It is therefore an object of the invention to provide an RF mixer device having one or more signal splitting/phase shifting element(s) (xe2x80x9cbalun(s)xe2x80x9d) integrated on-chip with the core RF mixer circuit elements. It is a further object of the invention to provide methods for making an RF mixer device comprising one or more baluns integrated on-chip with the core RF mixer circuit elements.
In one embodiment, the invention provides an RF mixer device including a plurality of transistors formed on a highly insulating substrate and at least one balun integrated on-chip with the transistors.
In a further embodiment, an RF mixer device includes silicon-based CMOS circuitry integrated on a common substrate with one or more balun(s), wherein the common substrate includes a thin, highly crystalline silicon layer formed on the highly insulating substrate. In preferred embodiments, the highly insulating substrate is selected from transparent crystalline materials such as sapphire, spinel, etc.
In another embodiment, an RF mixer includes one or more RF circuit element(s) integrated on a common substrate with one or more balun(s), wherein the common substrate comprises an ultrathin silicon-on-sapphire (UTSi) substrate.
In another embodiment, the invention provides methods for fabricating an RF mixer including RF circuit elements integrated on-chip with one or more baluns on a common substrate, wherein the common substrate for integration comprises ultrathin silicon-on-sapphire. By the method of the invention, mixers along with associated balun circuitry may be fabricated in ultrathin silicon on sapphire, characterized by extremely high linearity and low loss.
In yet another embodiment, the invention provides a method for forming an RF mixer having at least one RF circuit element formed in a thin layer of silicon on a sapphire substrate, wherein at least one port of the RF circuit element is connected to a balun integrated on-chip with the RF circuit element. In a further embodiment, the RF circuit element includes MOS transistors.
In a further embodiment, the invention provides a method to fabricate a high linearity passive mixer comprising MOS transistors formed on an ultrathin silicon-on-sapphire substrate, wherein the mixers typically exhibit conversion loss of less than about 7 dB and an input IP3 of greater than about 30 dBm.